Method of reducing porosity in thermal spray coated and sintered articles

ABSTRACT

This invention relates to a method for sealing porosity of at least a portion of an outer porous surface of an article, said method comprising (i) applying a sealant solution on the outer porous surface of said article, (ii) infiltrating at least a portion of the outer porous surface with said sealant solution, and (iii) allowing the infiltrated sealant solution to react, thereby forming an infiltrated solid precipitate, said infiltrated solid precipitate sealing the porosity of at least a portion of the outer porous surface of said article. The method is useful, for example, in the protection of integrated circuit manufacturing equipment, internal chamber components, and electrostatic chuck manufacture.

FIELD OF THE INVENTION

This invention relates to a method for in situ precipitation frominfiltrated inorganic chemical salt solutions to form compatible solidcompounds within the interconnected pores of thermal spray coated orsintered articles. The method is useful, for example, in the protectionof integrated circuit manufacturing equipment, internal chambercomponents, and electrostatic chuck manufacture.

BACKGROUND OF THE INVENTION

Thermal spray coatings can be used for the protection of equipment andcomponents. used in corrosive environments. Such thermally sprayedcoatings are derived from the heating, acceleration of materials,originally in solid form as powder or wire, through a thermal spraydevice in which they are rendered semi-molten and deformable prior toimpact on a work piece surface. As such, they contain differing degreesof residual porosity which may range in size from the clearly visible tothe sub-micron. Much of the porosity is interconnected and formsnetworks of surface-to-substrate channels into which corrosive processgases may penetrate. Corrosion of the coating, or of the substrate ifpenetration is complete, is unacceptable in coated equipment used inintegrated circuit manufacture because of the generation of contaminantcorrosion products and particles.

U.S. Pat. No. 5,869,144 discloses a process for sealing the outer poroussurface of an article with a boron nitride-silicate-containing sealant.Articles, i.e., rolls, intended for use with molten zinc are coated witha protective layer such as tungsten carbide cobalt, alumina, zirconia ormolybdenum boride. The sealant is then deposited over the coating toprevent penetration of molten zinc to the substrate of the roll and alsoto minimize buildup of oxides and/or dross on the surface of the coatedroll from the molten zinc.

There is a need in the art for blocking or sealing the inter-connectedresidual micro-porosity in thermal spray coated and sintered articles,including metallic and carbide-based coatings operating at hightemperatures, but particularly those of the ceramic oxides, e.g., yttriaand alumina, to reduce the level of corrosive attack by processreagents. There also is a need for blocking or sealing the porosityespecially in coatings or sintered bodies used in equipment employed inintegrated circuit manufacture without using any hydrocarbon or otherresin systems, or without introducing any foreign metal cations, mobileor otherwise, or without causing any coloration or reduction inwhiteness of the coating.

SUMMARY OF THE INVENTION

This invention relates in part to a method for sealing porosity of atleast a portion of an outer porous surface of an article, said methodcomprising (i) applying a first solution on the outer porous surface ofsaid article, (ii) infiltrating at least a portion of the outer poroussurface with said first solution, (iii) applying a second solution onthe outer porous surface of said article, (iv) infiltrating at least aportion of the outer porous surface with said second solution, and (v)allowing the infiltrated first solution and the infiltrated secondsolution to react, thereby forming an infiltrated solid precipitate,said infiltrated solid precipitate sealing the porosity of at least aportion of the outer porous surface of said article.

This invention also relates in part to a method for sealing porosity ofat least a portion of an outer porous surface of an article, said methodcomprising (i) applying a sealant solution on the outer porous surfaceof said article, (ii) infiltrating at least a portion of the outerporous surface with said sealant solution, and (iii) allowing theinfiltrated sealant solution to react, thereby forming an infiltratedsolid precipitate, said infiltrated solid precipitate sealing theporosity of at least a portion of the outer porous surface of saidarticle.

This invention further relates in part to articles, e.g., integratedcircuit manufacturing equipment, deposition chamber components andelectrostatic chucks, having an outer porous surface in which at least aportion of said outer porous surface is sealed with a precipitatedsealant solution.

This invention yet further relates in part to articles prepared by themethod of this invention.

The method of this invention enables interconnected porosity in thermalspray coated and sintered articles to be sealed with white,microcrystalline deposits which contain no foreign metallic cations andno hydrocarbon or silicone resins and solvents. The method isparticularly compatible with high purity oxide ceramic coatings used inthe electronics industry because high purity chemicals can be used. Themethod can enable treated thermal spray coated and sintered articles toexceed by a factor of 4-5 the 4 hour penetration resistance requirementof a typical hydrochloric acid test used by the industry. Additionally,the method of this invention can improve the whiteness of thermallysprayed ceramic oxide coatings and sintered bodies which is importantfor yttria and alumina coatings.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with this invention, in-situ precipitation frominfiltrated inorganic chemical salt solutions is used to form compatiblesolid compounds within the interconnected pores in articles having athermally sprayed coating or a sintered article, thereby reducing oreliminating porosity-related problems such as penetration and corrosionby fluids, (liquids or gases) in the articles. The porosity relatedproblems are applicable to a variety of metal, metal alloy and ceramicproducts, particularly so in coatings and ceramics used in theelectronics industry in plasma and chemical vapor deposition chambersand in electrostatic chucks where the presence of mobile contaminantmetallic cations must be avoided. Porosity related problems also occurin anodization which contains inherent surface-to-substratemicroporosity in the form of vertical channels formed naturally duringanodic growth of the oxide surface film.

This invention provides a method for blocking or sealing theinter-connected residual micro-porosity in thermal spray coated andsintered articles, particularly those of the ceramic oxides, yttria andalumina, thereby reducing the level of corrosive attack by processreagents. This has been accomplished by the method of this inventionwithout using any hydrocarbon or other resin systems, withoutintroducing any foreign metal cations, mobile or otherwise, and withoutcausing any coloration or reduction in whiteness of the coating. Specialelectronics grade brushes with nylon bristles held ideally by molding,rather than by a potentially soluble epoxy resin in a polyethylene orpolypropylene handle, should be used to avoid sodium and othercontaminants.

As shown in the examples hereinbelow, corrosion tests were conducted onthe articles of this invention based on the resistance of a coating topenetration by an aqueous solution of hydrochloric acid. In differentforms, hydrochloric acid penetration tests are invariably part ofcustomer qualification requirements for coatings.

As indicated above, this invention relates to a method for sealingporosity of at least a portion of an outer porous surface of an article,said method comprising (i) applying a first solution on the outer poroussurface of said article, (ii) infiltrating at least a portion of theouter porous surface with said first solution, (iii) applying a secondsolution on the outer porous surface of said article, (iv) infiltratingat least a portion of the outer porous surface with said secondsolution, and (v) allowing the infiltrated first solution and theinfiltrated second solution to react, thereby forming an infiltratedsolid precipitate, said infiltrated solid precipitate sealing theporosity of at least a portion of the outer porous surface of saidarticle.

In an embodiment, this invention relates to a method for sealingporosity of at least a portion of an outer porous surface of an article,said method comprising (i) applying a sealant solution on the outerporous surface of said article, (ii) infiltrating at least a portion ofthe outer porous surface with said sealant solution, and (iii) allowingthe infiltrated sealant solution to react, thereby forming aninfiltrated solid precipitate, said infiltrated solid precipitatesealing the porosity of at least a portion of the outer porous surfaceof said article.

This invention consists of the application of solutions of compatibleinorganic ionic reagents such that they are infiltrated into theresidual porosity of coatings or sintered bodies and react chemicallyin-situ within the coating or sintered body to form a solid precipitate.The formation of these solid precipitates seals porosity from within thebody of the coating.

The method of this invention uses inorganic ionic solutions ofcompatible chemical reagents. For example, the method uses yttrium saltsfor yttrium coatings, aluminum salts for alumina coatings and can usezirconium salts for zirconia coatings. This invention can employvirtually any inorganic ionic solution with any coating or sintered bodyand the solutions are interchangeable, e.g., cerium salts with yttriacoatings, aluminum salts with zirconia coatings, and the like. Thereacting solutions, e.g., a first solution comprising a metal saltsolution (e.g., yttrium nitrate) and a second solution comprising abasic solution (e.g., ammonium hydroxide) or a sealant solutioncomprising a mixture of the metal salt solution and basic solution, arepreferably of low viscosity, thereby allowing the solutions to penetratethe micron and sub-micron porosity found in thermally sprayed coatingsand sintered ceramic bodies.

The yttrium and zirconium salts work well separately and together toform hydroxy-nitrate precipitated solid compounds, and work better ifthese reaction products are fired after drying. Since most chambercomponents for integrated circuit manufacture are made from aluminum,the scope for high temperature heat treatment is limited to about 200°C. For high temperature thermal barrier coatings on nickel alloys forgas turbines or for corrosion resistant coatings on steels for thegalvanizing industry, the in-situ precipitates of zirconia and yttriacan easily be heat treated to achieve decomposition into oxides. Thescope for doping zirconia coatings with other oxides by this method isvery wide because of the scope for thermal decomposition allowed by theuse of nickel and cobalt super alloy substrates.

In an embodiment, a preferred method of this invention includes apost-application heat treatment of the sealed article at a temperatureof greater than 750° C., (rather than a drying treatment at 120° C.dictated by the presence of an aluminum substrate,) in order tostabilize, rather than dry, the in-situ precipitate.

In the method of this invention, a sealing treatment agent is appliedonto the thermal sprayed coating formed on the surface of the substrateor onto the sintered article in the aforementioned coating formationstep. The sealing treatment agents are selected from solutions ofinorganic salts of a desired metallic cation, which can form solidreaction products on contact. In an embodiment employing a first andsecond solution, the solutions are applied sequentially, allowed to comeinto contact and to react chemically within the pores of the porousarticle. In an embodiment employing a sealant solution, although somereaction will have started upon the mixing of the metal salt solutionwith the base solution, the sealant solution should be applied insufficient time to allow infiltration and precipitation of the solutionwithin the pores of the porous article.

Illustrative metal salts which can be used in the method of thisinvention include, for example, yttrium, cerium, lanthanum, aluminum orother nitrates, chlorides or sulfates. Nitrates are particularlyeffective in electronic applications since they avoid contamination bysulfur and chlorine. Illustrative basic reacting solutions include, forexample, ammonium hydroxide, sodium hydroxide and potassium hydroxide.For certain electronic applications, basic materials such as sodiumhydroxide and potassium hydroxide are highly undesirable because of thehigh mobility of sodium and potassium cations and process chambercontamination issues.

The concentration of the metal salt starting solution can vary over awide range, and need only be that minimum amount necessary to react withthe base starting material to form the hydroxy-nitrate precipitates. Ingeneral, the metal salt starting material concentrations in the range offrom about 1 millimole or less to about 10,000 millimoles or greater,should be sufficient for most processes.

The concentration of the base starting solution can vary over a widerange, and need only be that minimum amount necessary to react with themetal salt starting material to form the hydroxy-nitrate precipitates.In general, the base starting material concentrations in the range offrom about 1 millimole or less to about 10,000 millimoles or greater,should be sufficient for most processes.

Typically, the aqueous sealant solutions can contain from about 20 toabout 300 grams per liter of yttrium nitrate, and from about 20 to about200 milliliters per liter of ammonium hydroxide (0.880 specificgravity). Preferably, the aqueous sealant solution can contain fromabout 80 to about 140 grams per liter of yttrium nitrate, and from about25 to about 40 milliliters per liter of ammonium hydroxide. It is alsowithin the scope of this invention to use solutions of two differentmetallic salts, such as a mixture of yttrium and zirconium or magnesium.

The reacting sealant solution, e.g., yttrium nitrate and ammoniumhydroxide, after chemical reaction forms hydroxy-nitrate precipitates,which can provide excellent resistance to penetration and corrosion byfluids, e.g., liquids and gases, especially gas corrosion. The reactivesealant solutions are preferably applied to the coated or sinteredarticle which is used in an environment of corrosive gases. Corrosivegases and liquids attack coated articles, sintered articles and the likeand easily penetrate into small holes or gaps in the micrometer rangebecause of their porosity.

Illustrative hydroxy-nitrate precipitates used in the method of thisinvention include, for example, yttrium hydroxy nitrate, cerium hydroxynitrate, lanthanum hydroxy nitrate, and the like.

Reaction conditions for the reaction of the base material, e.g.,ammonium hydroxide, with the metal salt material, e.g., yttrium nitrate,such as temperature, infiltration time and contact time, may also varygreatly and any suitable combination of such conditions may be employedherein. Normally the reaction is carried out under ambient temperatureand the infiltration and contact time may vary from a matter of secondsor minutes to a few hours or greater. The reactants can be applied tothe coated or sintered article separately in any order or combined inany order. For sealant solutions, although some reaction will havestarted upon the mixing of the metal salt solution with the basesolution, the mixing time employed should be sufficiently short to allowthe sealant solution to be applied and infiltrated into the coated orsintered article.

For electronics applications, electronics grade chemical reagents areessential to avoid unacceptable levels of mobile metallic cations in theprecipitate. In an embodiment of this invention, the application ofammonium hydroxide followed by the appropriate yttrium or aluminum saltssolution gives more consistent hydrochloric acid test performance thatthe other way round.

According to this invention, reactive sealant solutions are provided forthermally sprayed coated articles or sintered articles intended to comeinto contact with a corrosive environment. The precipitated sealantsolution reduces or eliminates porosity-related problems such aspenetration and corrosion by fluids, particularly in coatings andceramics used in the electronics industry in plasma and chemical vapordeposition chambers and in electrostatic chucks where the presence ofmobile contaminant metallic cations must be avoided. The sealantsolution is easy to apply and cost effective to produce.

The sealing material exhibits desired resistance to corrosive liquidsand gases, thus making it ideally suitable for coating structuralmaterials, such as components used in the plasma chambers employed inthe manufacture of integrated circuit components, internal depositionchamber components, electrostatic chucks and the like, that are intendedto be used in or in contact with corrosive environments.

An illustrative sealing method of this invention is as follows:

(a) preparing a first reactive solution;

(b) applying the first reactive solution on the outer porous surface ofthe article to be sealed;

(c) allowing the first reactive solution to infiltrate the outer poroussurface of the article;

(d) applying a second reactive solution on the outer porous surface ofthe article to be sealed;

(e) allowing the second reactive solution to infiltrate the outer poroussurface of the article;

(f) allowing the first and second reactive infiltrated solutions toreact, thereby forming an infiltrated solid precipitate, saidinfiltrated solid precipitate sealing the porosity of at least a portionof the outer porous surface of said article; and

(g) heating the coated article in an appropriate temperature range tosubstantially remove the water from the article and, if the temperatureis high enough, convert the infiltrated solid precipitate to an oxide.

Accordingly, this invention utilizes a sealant having an excellentresistance to corrosive fluids, especially to corrosive gases, and thesealant minimizes or eliminates contamination of the coated or sinteredarticle. The sealant comprises an aqueous solution which can be appliedto the surface of an article by painting, spraying, such as thermalspraying, or using any other conventional technique.

After applying the aqueous solutions to the article, it should beallowed to infiltrate the porous surface of the article and then driedto remove substantially all of the water. Preferably, the water in thecoating should be reduced to 10% or less of the water used in theaqueous solution and preferably reduced to 5% or less of the water usedin the aqueous solution. To insure removal of the water, the coatedarticle could be heated above 100° C. for a time period to reduce thewater in the coating to 5% or less. Generally, a time period of about 4to about 8 hours would be sufficient, with a time period of about 16 toabout 24 hours being preferred. It is preferable to heat the coatedarticle above 100° C. since chemically combined water is removed betterat higher temperatures.

The sealant will eliminate or reduce porosity related problems such aspenetration and corrosion of the porous surface by liquids and gases.The amount of yttrium salts should be sufficient to provide a compatiblesolid compound infiltrated into the porous surface, thus sealing thearticle from penetration and corrosion by fluids.

According to this invention, articles intended for use in corrosiveenvironments are first thermal spray coated with a protective coatinglayer or a sintered article is produced. The sealant can then bedeposited over the coating or sintered article to prevent penetration ofcorrosive liquids and gases to the substrate of the article. For sealingporosity in a zirconia outer porous surface of an article byprecipitation of a zirconium compound, a solution of zirconium nitrateand a solution of ammonium hydroxide could be infiltrated andprecipitated in the porosity of the outer porous surface. The sealedthermal sprayed coated article or sintered article formed by the methodof this invention may have desired corrosion resistance, heatresistance, thermal shock resistance, oxidation resistance, and wearresistance.

The method of this invention offers significant advantages, inparticular economic advantages. It uses high purity reagents which arecommercially available from laboratory suppliers for analytical andelectronics industry purposes. It is an ambient laboratory or clean-roommethod requiring no vacuum chambers or capital equipment other thanstandard chemical laboratory equipment. The chemical regents areinorganic, inexpensive and with minimal handling and usage hazards. Theyhave low level corrosive and oxidizer classifications. With respect tothe reagents used in the method, there are no exceptional disposalprocedures involved.

In cases where only certain parts of a component are coated, the methodof this invention may work best by applicator painting. High gradecleaned and boiled electronics grade brushes should be used to avoidcontamination. Tank immersion would lead to cross contamination.

There is no real size limitation on articles which may be treated by themethod of this invention, provided that the articles can be dried in anair oven as described or at temperatures of up to 100° C. if necessary.It would be desirable to heat treat the internal precipitate to ensuredecomposition, but this is not practicable with an aluminum substrate. Arastering laser may work with an aluminum substrate. Materials otherthan aluminum will almost certainly have higher temperature capability.It is also important not to leave residual pools or puddles betweentreatments because once they begin to dry, they leave water marks whichcan be difficult to remove.

The coated articles used in the method of this invention can be preparedby flowing powder through a thermal spraying device that heats andaccelerates the powder onto a base (substrate). Upon impact, the heatedparticle deforms resulting in a thermal sprayed lamella or splat.Overlapping splats make up the coating structure. A plasma spray processuseful in this invention is disclosed in U.S. Pat. No. 3,016,447, thedisclosure of which is incorporated herein by reference. A detonationprocess useful in this invention is disclosed in U.S. Pat. Nos.4,519,840 and 4,626,476, the disclosures of which are incorporatedherein by reference, which include coatings containing tungsten carbidecobalt chromium compositions. U.S. Pat. No. 6,503,290, the disclosure ofwhich is incorporated herein by reference, discloses a high velocityoxygen fuel process useful in this invention to coat compositionscontaining W, C, Co, and Cr. Cold spraying methods known in the art mayalso be useful in this invention. Typically, such cold spraying methodsuse liquid helium gas which is expanded through a nozzle and allowed toentrain powder particles. The entrained powder particles are thenaccelerated to impact upon a suitably positioned workpiece.

In coating the articles of this invention, the thermal spraying powderis thermally sprayed onto the surface of the article, and as a result, athermal sprayed coating is formed on the surface of the article.High-velocity-oxygen-fuel or detonation gun spraying are illustrativemethods of thermally spraying the thermal spraying powder. Other coatingformation processes include plasma spraying, plasma transfer arc (PTA),or flame spraying. For electronics applications, plasma spraying ispreferred for yttria and alumina coatings because there is nohydrocarbon combustion and therefore no source of contamination. Plasmaspraying uses clean electrical energy. Preferred coatings for thermallyspray coated articles of this invention include, for example, yttriumoxide, zirconium oxide, magnesium oxide, cerium oxide, aluminum oxide,or oxides of Groups 2A to 8B inclusive of the Periodic Table and theLanthanide elements.

The method of this invention can be used with pressed and sinteredalumina and yttria and ceramic alloys and other coatings such asalumina-titania, and multi-layered sprayed ceramic coatings. The methodmay be suitable for forming metal-ceramic composites where ceramicloadings are low, and it may even be useful in toughening ceramics byproviding energy absorbing microcrystalline crack-stoppers.

This method appears readily adaptable to the precipitation of inorganiccompounds of other materials of interest in ceramic coatings forelectronics applications such as ceria, magnesia and hafnia in ceria,magnesia or hafnia thermally sprayed coatings or the creation of, forexample, ceria sealing precipitates in an yttria coating or possiblytitania in an yttria or hafnia coating. Many combinations are includedwithin the scope of this invention if the appropriate inorganic reagentsare used.

The in-situ precipitation of desired compounds within the pores of aporous body can also be applied to bind solutions of nanoparticle orslurries which are infiltrated. As an example, a fluid suspension ofnanoparticles of yttrium oxide is infiltrated into the porosity of athermally sprayed oxide ceramic coating and the suspension fluid isallowed to dry by evaporation. Two reacting solutions, e.g., yttriumnitrate and ammonium hydroxide, can be infiltrated sequentially andallowed to react chemically. The resultant precipitated material bindsin the infiltrated nanoparticles thereby blocking or closing theporosity and preventing the nanoparticle from becoming dislodged.

Slurries of suspended particles, e.g., alumina, may also be infiltratedinto a porous body and bonded in place by use of aluminum based reactingsolutions or by solutions or another material of choice, e.g., yttriumnitrate, cerium nitrate or zirconium nitrate.

The inherent pore channels in anodization may be treated with solutionsof metallic salts as described herein to enhance the corrosionresistance of anodically grown alumina film.

In another embodiment, in view of the very complex geometries of partswhich require coating with a line of sight thermal spray process, themethod of this invention may be useful for making a “paint-on” coatingor for supplementing the thermal spray coating in some way so as toafford protection to difficult areas or areas where, for example,anodizing ends and thermal spray coating begins.

It should be apparent to those skilled in the art that this inventionmay be embodied in many other specific forms without departing from thespirit of scope of the invention.

The following examples are provided to further describe the invention.The examples are intended to be illustrative in nature and are not to beconstrued as limiting the scope of the invention.

EXAMPLES

Aqueous solutions of high purity yttrium nitrate and ammonium hydroxidewere prepared in several different concentrations and applied indifferent orders in various examples to thermally sprayed yttrium oxidecoated samples. Ammonium hydroxide reactant was applied first andallowed to soak into the coating without leaving puddles on the surfacefollowed by the yttrium nitrate solution, applied similarly. Residualpuddles or pools left water marks which could not be removed. A reactiontime of 1 hour in air at room temperature was allowed after which thetest pieces were air-oven dried overnight (12 or more hours).

The samples were whiter in color after treatment, attributed to improvedreflectivity due to porosity closure and no effects of the treatmentwere visible on the sample surfaces. The samples were set up for 5weight percent aqueous hydrochloric acid penetration test and testingcommenced. The hydrochloric acid penetration test is described below.

Previous tests on untreated thermally sprayed coatings had given timesto penetration of 5 to 25 minutes and a sample of anodized aluminum hadlasted for 3.5 hours. Samples treated with the yttrium salts lasted20-24 hours which compared favorably with the normal targets for thistype of test of 4-8 hours. An unexpected result was that the yttriacoating itself (0.006 inches) was consumed after 24 hours with the earlystages of aluminum substrate grain boundary attack just visible inmetallurgical cross-section at 200×.

As used in the examples, a hydrochloric acid penetration test wasconducted to assess and rank the quality of thermally sprayed ceramiccoatings, particularly yttria, in terms of the time to penetration by acontacting aqueous solution of hydrochloric acid. Time to penetrationwas taken as the elapsed time between introduction of the acid andpenetration of the coating to an aluminum substrate such that substratecorrosion by the acid led to the generation of a stream of at least 2bubbles of hydrogen gas per second. This point was determined visually.Coatings were ranked in terms of time to penetration.

The acid used in the hydrochloric acid penetration test was a 5 weightpercent aqueous solution of concentrated laboratory hydrochloric acid ofspecific gravity 1.19 grams per milliliter, which represents a fullysaturated solution of hydrogen chloride gas in water at ambienttemperature and pressure, i.e., fully concentrated hydrochloric acid.

To make a litre of a 5 weight percent aqueous solution of concentratedhydrochloric acid as used in the hydrochloric acid penetration tests,42.3 milliliters of concentrated Analytical Grade hydrochloric acid ofspecific gravity 1.19 grams per milliliter acid was added to 957.7milliliters of deionized water of resistivity greater than 3 megOhm.

The hydrochloric acid penetration test was carried out under ambient labconditions 20-30° C. Samples were handled using latex gloves and wipedwith approved clean-room wipes. Washing was carried out with deionizedwater and with approved isopropyl alcohol. Glass cylinders and any otherequipment and materials which may come into contact with the coating orreagents used in-the test were prepared by washing with deionized water,wiping with clean-room wipes and drying with isopropyl alcohol beforeuse.

The procedure for carrying out the hydrochloric acid penetration test isas follows:

Take a 4 inch×4 inch×0.13 inch thermally spray coated test panel couponand attach to its coated surface at least one clear acid-resistant glasscylinder using an acid-resistant sealant glue. To do this run a bead ofsealant around the base of the cylinder at its contact point with thecoated sample taking care not to allow sealant to run under the cylinderwalls and on to the coated surface to be tested. With carefulpositioning it may be possible to attach 4 or even 5. Each cylindershould have a minimum inner diameter of 0.5 inch and a minimum height of3 inches.

Allow the acid-resistant sealant glue to cure.

Prepare 5 weight percent aqueous solution of hydrochloric acid as above,(42.3 milliliters of concentrated analytical grade hydrochloric acidadded to 957.7 milliliters of deionized water greater than 3 megOhm).

Position a black matte plane surface behind the array of cylinders andarrange a light source at right angles to the direction of viewing. Thisshows the evolution of fine bubbles more clearly than direct observationunder normal lab. lighting conditions.

Fill the cylinders with the 5 weight percent aqueous hydrochloric acidsolution to a minimum depth of 2 inches.

Observe the coating surface and the solution in the cylinders and recordthe times taken for each sample to generate a stream of hydrogen gasbubbles with an evolution of 2 bubbles per second.

Acceptance criteria vary but a typical electronics industryspecification requires that a coupon not show an evolution of 2 hydrogengas bubbles per second for at least 4 hours, i.e., the time topenetration of the acid should be at least 4 hours.

Under the above test conditions, panels thermally sprayed with yttriacoatings of 0.008 inches thickness showed times to penetration in therange of 1 to 2.5 hours. After treatment with the sealant solutiondescribed herein, times to penetration of coatings of the same thicknesswere increased to between 12 and 16 hours. In several cases, penetrationwas delayed until total consumption of the coating in contact with theacid had occurred which gave penetration times of more than 20 hours.This was confirmed by examination of a metallurgical cross-section whichshowed early stages of inter-granular corrosion penetration.

Samples were tested for low particle generation, and helium pycnometryshowed reductions in sub-micron porosity from 17 percent to 5 percent.

Yttrium nitrate, analytical grade, was made up into a 0.3 molar, (80grams/liter) aqueous solution in 8 meg ohm de-ionized water and a 10milliliters/litre solution of ammonium hydroxide was made up. The formeris designated “Solution A” and the latter, “Solution B”. Observations ofprecipitation on a watch glass showed that precipitation started afterabout 2 minutes which gave enough time for the solutions to soak intothe yttria ceramic coating.

In one experiment, Solution A was applied to the yttria coating on a 4inch×4 inch aluminum sheet and after allowing 2 minutes of penetrationtime and blotting off any excess surface solution, Solution B wasapplied. After 20 minutes reaction time, the sample was air oven driedat 50° C. for 12 hours. This sample resisted penetration in thehydrochloric acid test for more than 8 hours.

In another test, the application of solutions was reversed and SolutionB was applied to a similar sample, allowed to soak in for 2 minutes,blotted off and then Solution A was applied. Watch glass tests showedprecipitation to be denser than before and so this is the preferredorder. The sample coated with solutions in the order Solution B thenSolution A resisted penetration in the hydrochloric acid test for 20hours. An unexpected result, obtained after metallurgical examination ofthe sample in cross section, was that, after this time, the fullthickness of yttria coating, 0.008 inches, 200 micrometers, was consumeddown to the aluminum substrate.

Further experiments to increase the air oven drying temperature werecarried out but only to about 105° C. because of concerns that thecoating might separate from the aluminum substrate due to thermalexpansion mismatch.

X-Ray diffraction analysis was run on samples prepared by the abovemethods because there was concern about the undesirable presence ofmobile metallic cations. The only lines observed on the patterns weredue to yttrium oxide, yttrium hydroxide and yttrium nitrate hydrate.

For a zirconia coating desired to be sealed on a high temperaturesubstrate material such as a steel, nickel or cobalt based alloy, withan in-situ precipitate of zirconium oxide or yttrium oxide, thetemperature capability of the substrate allows the precipitate to beheat treated above 800° C. The product would be expected to be acomposite of yttrium oxide grains bonded by zirconia.

A further experiment showed that the addition of 50 grams per liter ofaluminum nitrate to reacting solutions of yttrium nitrate and ammoniumhydroxide after firing at high temperature, would be expected to giveyttrium oxide grains bonded by alumina.

The low melting point of aluminum alloys precludes the benefits of thesereactions because of limitations on heat treatment temperature.

1. A method for sealing porosity of at least a portion of an outerporous surface of an article, said method comprising (i) applying afirst solution on the outer porous surface of said article, (ii)infiltrating at least a portion of the outer porous surface with saidfirst solution, (iii) applying a second solution on the outer poroussurface of said article, (iv) infiltrating at least a portion of theouter porous surface with said second solution, and (v) allowing theinfiltrated first solution and the infiltrated second solution to react,thereby forming an infiltrated solid precipitate, said infiltrated solidprecipitate sealing the porosity of at least a portion of the outerporous surface of said article.
 2. The method of claim 1 furthercomprising heating said article after forming the infiltrated solidprecipitate.
 3. The method of claim 1 wherein (i) the first solutioncomprises a basic solution and the second solution comprises a metalsalt solution, or (ii) the first solution comprises a metal saltsolution and the second solution comprises a basic solution.
 4. Themethod of claim 1 wherein (i) the first solution comprises a metal saltselected from yttrium nitrate, cerium nitrate or lanthanum nitrate andthe second solution comprises a base selected from ammonium hydroxide,potassium hydroxide or sodium hydroxide, or (ii) the first solutioncomprises a base selected from ammonium hydroxide, potassium hydroxideor sodium hydroxide and the second solution comprises a metal saltselected from yttrium nitrate, cerium nitrate or lanthanum nitrate. 5.The method of claim 1 wherein the infiltrated solid precipitatecomprises yttrium hydroxy nitrate, cerium hydroxy nitrate or lanthanumhydroxy nitrate.
 6. The method of claim 1 wherein the article comprisesa metallic article with a thermally sprayed coating or a sintered porousmetallic or ceramic article.
 7. The method of claim 1 wherein thearticle comprises a chamber or a component of said chamber used in theproduction of an integrated circuit component.
 8. An article prepared bythe method of claim
 1. 9. A method for sealing porosity of at least aportion of an outer porous surface of an article, said method comprising(i) applying a sealant solution on the outer porous surface of saidarticle, (ii) infiltrating at least a portion of the outer poroussurface with said sealant solution, and (iii) allowing the infiltratedsealant solution to react, thereby forming an infiltrated solidprecipitate, said infiltrated solid precipitate sealing the porosity ofat least a portion of the outer porous surface of said article.
 10. Themethod of claim 9 further comprising heating said article after formingthe infiltrated solid precipitate.
 11. The method of claim 9 wherein thesealant solution comprises a metal salt solution and a basic solution.12. The method of claim 9 wherein the sealant solution comprises a metalsalt selected from yttrium nitrate, cerium nitrate or lanthanum nitrate,and a base selected from ammonium hydroxide, potassium hydroxide orsodium hydroxide.
 13. The method of claim 9 wherein the infiltratedsolid precipitate comprises yttrium hydroxy nitrate, cerium hydroxynitrate or lanthanum hydroxy nitrate.
 14. The method of claim 9 whereinthe article comprises a metallic article with a thermally sprayedcoating or a sintered porous metallic or ceramic article.
 15. The methodof claim 9 wherein the article comprises a chamber or a component ofsaid chamber used in the production of an integrated circuit component.16. An article prepared by the method of claim
 9. 17. An article havingan outer porous surface in which at least a portion of said outer poroussurface is sealed with a sealant comprising the reaction product of ametal salt and a base.
 18. The article of claim 17 wherein the metalsalt is selected from yttrium nitrate, cerium nitrate or lanthanumnitrate and the base is selected from ammonium hydroxide, potassiumhydroxide or sodium hydroxide.
 19. The article of claim 17 comprising ametallic article with a thermally sprayed coating or a sintered porousmetallic or ceramic article.
 20. The article of claim 17 comprising achamber or a component of said chamber used in the production of anintegrated circuit component.
 21. The article of claim 17 whichcomprises a thermally spray coated article or a sintered article. 22.The article of claim 21 wherein the thermally spray coated article iscoated with yttrium oxide, zirconium oxide, magnesium oxide, ceriumoxide, aluminum oxide, or oxides of Groups 2A to 8B inclusive of thePeriodic Table and the Lanthanide elements.
 23. The article of claim 21wherein the thermally spray coated article has a porosity of not greaterthan about 5.0%.
 24. The article of claim 17 further comprisingnanoparticles or slurries bonded to an infiltrated solid precipitate,said infiltrated solid precipitate sealing the porosity of at least aportion of the outer porous surface of said article.